Genetic components in a thermal developmental plasticity of the beetle Tribolium castaneum

Low developmental temperatures cause ectotherms to retard growth, postpone maturation, and emerge at either larger or smaller adult size. In this study, we explored how these thermal responses evolved, focusing on their genetic basis. We applied a full diallel breeding design on inbred lines of the flour beetle, Tribolium castaneum. To assess the proportional contributions of genetic and non-genetic effects, each genotype, a unique combination of parental haplotypes, was reared from an egg to imago at five developmental temperatures. Faster development of females vs. males was associated with comparatively larger body masses of females (pupae and imago). In contrast, the rapid development caused by warmer environments resulted in smaller beetles (pupae and imago), but there were significant differences in this trait among genotypes. Independent effects of parental haplotypes played the major role in explaining the variance of body mass, but interactive effects of parental haplotypes explained most of the variance in developmental length. Genotypes responded to the thermal environment in a markedly uniform way. Nevertheless, we found the low statistically significant variance in the slopes of thermal reaction norms for body mass and developmental, which was mainly driven by the interactive effects of parental haplotypes. Overall, the thermal plasticity of T. castaneum follows the most common pattern among ectotherms, the so-called temperature-size rule. Detection of the low genetic variance in the shape of this response supports the idea that thermal developmental plasticity remains under a strong selective pressure in ectotherms.